and y-) and Starch - EPrints USM

I '

Transport Method for Determining the Stability Constants of Complexes with Cyclodextrins (a-,

~-

and

y-)

and Starch as Hosts, Molecular Iodine and Triiodide Anion as Guests in Water

r,,},

^{I.K) ~C"-I.9}

BO-LONG POH* and CHIN LING LOH

School alChemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia

ABSTRACT

A transport method was introduced to determine the stability constants of the 1: 1 complexes formed between cyclodextrins (a-, r3- and y-) as hosts and molecular iodine and triiodide anion as guests in an aqueous medium. The stability constants obtained when the guest is iodine are 3.7 x 10^3, 2.4 ^X 10² and 1.4 x 10² M-¹ at 27°C for a-, r3- and y- cyclodextrin respectively. When the guest is triiodide anion, the stability constants obtained are 4.1 x 10³, 7.2 X 10² and 2.2 x 10² M-¹ respectively for a-, r3- and y-cyclodextrin. No complexation between starch and iodine was observed. However, the starch molecule was induced into a folded conformation resembling r3-cyclodextrin to complex wth triiodide anion, with a stability constant of7.9 x 10²M-¹.

Key words: cyclodextrin-iodine complex, cyclodextrin-triiodide amon complex, starch- triiodide anion complex, stability constant, transport method

(Paper presented at the 39^th IUPAC Congress and 86^th Conference of the Canadian Society for Chemistry, August 10-15,2003, Ottawa, Canada).

1.INTRODUCTION

Cyclodextrins (a-, _~- and y-) form complexes with both molecular iodine and triiodide anion and they are good models for the starch-iodine complex. The complexes formed in an aqueous medium are of 1: 1 host to guest stoichiometry [1,2]. Their stability constants have been determined by spectrophotometric [1], potentiometric [3], solubility [4] and volatization [5] methods.

In this paper, we introduce a transport method of determining the stability constants of these complexes and use it to gain an understanding of the conformation of the starch- iodine complex.

2. EXPERIMENTAL

2.1 Materials

a-, ~- and y-cyclodextrin (Sigma), iodine, potassium iodide and starch (Fisher) were commercial samples.

2.2 Visible spectra

Visible spectra of iodine in n-hexane solution were recorded with a Hitachi U-2000 spectrophotometer. The absorbance maximum at 522 nm (8 915) was used for quantitative determination of iodine.

2.3 Transport method

Transport experiments were carried out at room temperature (27±1°C) using the V-tube (Figure 1) described by Diederich and Dick [6,7]. A 16 mL aqueous solution of the host (1.0 x 10-³to 2.0 x 10-³ M for the cyclodextrins and 0.8 to 2.8 gIL for starch) was placed at the bottom of the V-tube. For experiments carried out in the presence of iodide anion, 1.0 x 10-² M of potassium iodide was added to the aqueous phase. Atop this aqueous phase in one arm of the tube was placed 8 mL of n-hexane containing iodine (1.0 x 10-³ M) as the source phase. In the other arm 8 mL of n-hexane was placed atop the aqueous phase as the receiving phase. The aqueous phase was stirred with a small magnetic bar at a constant speed. An identical experiment, but without the host, was used as the blank.

Samples of 2.5 mL were taken from both the source and receiving phases (uniform in concentration) at regular time intervals of about four hours or more for the quantitative determination of iodine in these two phases using the visible absorbance at 522 om. After each determination (less than a minute taken) the sample was returned to the two arms of the V-tube. No significant error due to sampling was observed. For each host, experiments were carried out with four different host concentrations and in duplicates.

There was negligible hydrolysis of iodine to iodide anion in the aqueous phase in the duration of the experiments, consistent with the reported small iodine hydrolysis equilibrium constant [8]. A representative sample of the raw data obtained is shown in Table 1.

2.4 Stability constants

For the complexation of iodine in the aqueous phase of the V-tube, the stability constant K1of the 1: 1 host (H) to guest (Iz) complex (H.Iz) is given by Equation (1).

K1= [H.h]/[H][ h] (1)

Using the assumption that the rate of diffusion of iodine from the hexane source phase into the aqueous phase containing the host is the sum of the iodine diffusion rate for the blank and that caused by complexation with the host [7], the term [h] is equal to the concentration of iodine in the aqueous phase of the blank U-tube and calculated by Equation (2).

[h]= {[h]o - [hls - [h]R}S/2 (2)

The three terms [h]o, [h]s and [h]R refer to the initial concentration of iodine in the source phase, the concentration of iodine in the source phase at time t and the concentration of iodine in the receiving phase at time t respectively. The subscript B refers to the blank U-tube. The dilution factor 2 takes into account the volume of the aqueous phase is twice that of the source and receiving phase.

The term [H.h] is calculated from Equation (3) where [h]H (the subscript H refers to the host U-tube) is obtained from the U-tube containing the host using Equation (4) which is analogous to Equation (2).

[H.Iz] ^{= [h]H - [h] (3)}

The term [H] is calculated from Equation (5) where [H]o refers to the initial concentration of the host.

[H] [H]o - [H.h] (5)

A representative example showing the calculations of Kl using Equation (l) for a- cyc10dextrin as host is shown in Table 2.

When triiodide anion is the guest, the stability constant K2 for the 1: 1 complex is given by Equation (6).

(6)

Under the present experimental conditions, any iodine molecule from the hexane source phase that gets into the aqueous phase is practically converted to the triiodide anion by the iodide anion present in the aqueous phase since the equilibrium constant between iodine and triiodide anion is about 700 M-¹ [4]. Therefore, replacing [h] with [13-] and [h]H with [13-]H in all the above equations for the case of iodine as guest we can obtain [H.I3-] and calculate K_{2 .} A representative example showing the calculations of K2for a-

cyc10dextrin as host is shown in Table 3.

3 RESULTS AND DISCUSSION

3.1 Complexation with iodine

Figure 2 is a plot

of

the absorbance of ^iodine in the hexane source phase (with a host in the aqueous phase) against the absorbance of iodine in the hexane source phase for the blank, taken at various time intervals. A host that can complex with iodine in the aqueous phase induces a faster flow ofiodine from the hexane source phase to the aqueous ^phase, resulting in a larger decrease in the iodine absorbance. Figure 2 shows that all the three cyclodextrins complex with iodine, the strongest complexation is with a-cyclodextrin and

the weakest complexation with y-cyclodextrin. Starch does not have any noticeable complexation with iodine, as its plot falls on the line with no complexation (the dotted line of unit slope) and the characteristic blue color for the starch-iodine complex was not observed

in

the aqueous phase. The absence

of

any starch-iodine complexation in the absence of iodide anion is consistent with earlier reports [1,9].

The values

of

the stability constant

Kr of

the 1:1 cyclodextrin-iodine complexes (Table 4) obtained from Equation (1) are of the same magnitude as those obtained by other methods, thus giving a strong support to the validity of our method.

3.2 Complexation with triiodide anion

Figure 3 is a plot

of

the absorbance of iodine in the hexane source phase (with a host and potassium iodide in the aqueous phase) against the absorbance of iodine in the hexane source phase for the blank (containing only potassium iodide in the aqueous phase), taken

at various time intervals. The three cyclodextrins as well as starch complex with the triiodide anion (the dotted line of unit slope is for no complexation). The plot for starch falls between the plots for a- and B-cyclodextrin, indicating ^the cavity size of the induced

folded conformation

of

starch enclosing the triiodide anion

is

bigger than that

of

a- cyclodextrin but smaller than that

of

B-cyclodextrin. For such

a

size, CPK ^molecular

models indicate that the folded conformation of starch consists of seven glucose units, not all lying on the same plane.

Thoma and French [1] ^as well as Diard and coworkers [3] ^{reported that} a-cyclodextrin binds with triiodide anion rather than iodide anion whereas Sano and coworkers _[10]

reported the reverse. Our results shown

in

Figure

3

^support the former. Preferential complexation with triiodide anion results

in

a larger decrease

of

iodine

in

the hexane source phase compared with the blank, as observed

in

Figure

3,

whereas preferential complexation with iodide anion does not since iodide anion is already present in large excess

in

the aqueous phase.

The values of the stability constant Kz of the

l:1

cyclodextrin-triiodide anion complexes (Table 5) were obtained from Equation (6). Treating the starch molecule as a series

of

^p-

cyclodextrin to obtain [H] ^{for starch,} the Kz value of 7.9 x 102

M'l

was similarly obtained

for the starch-triiodide anion complex. The Kz values are lower, especially that of a- cyclodextrin as host, than those reported in the literature [3].

ACKNOWLEDGEMENT

We thank the Malaysian Govemment for ^a R & D Grant to carry out this research.

REFERENCES

1.

Thoma, J.A.; French, D.

J

Am. Chem. Soc., 1958, 80, 6142-6146.

2.

Thoma, J.A.; French,D. J. Phy. Chem., 1958, 62,1603.

3. Diard, J.P.;

Saint-Aman,

E.;

Serve,

D.

Electroanal. Chem. Interfacial Electrochem., 1985, I 89,

II3-I20.

4.

^Sanemasa, I.; Kobayashi,

T.;

Deguchi, T. Bull. Chem. Soc. Jpn.,1985,

58,

1033- 1036.

5.

^Sanemasa,

I.;

Nishimoto,

Y.;

Tanaka,

A.;

Deguchi,

T.

Bull. Chem. Soc. Jpn., 1986, 59,2269-2272.

6.

Diederich, F.;

K.Dick.l

Am. Chem. 9oc,1984, 106,8024-8036.

7.

^Poh, B.-L.; Lim, C.S.; Koay, L.-5. Tetrahedron, 1990, 46,6155-6160.

8.

^Burger, J.D.; Liebhafsky, H.A. Anal. Chem.,1973, 45,600-602.

9.

^Banks, W.; Greenwood, C.T. Starch and its Componenrs, Edinburgh University Press, U.K., 1995, pp 95 and

l0l.

10. Sano, T.; Yamamoto, M.; Hori, H.; Yasunaga, T. Bull. Chem. Soc. Jpn.,1984,57, 678-680.

TABLE

1. Variation

of

iodine concentrations

in

the hexane source and receiving phases with time

(hour)

Source phase,

M

Receiving phase, M Source phase,

M

Receiving phase, M

3.8

8.3

1.016 x 10-'

8.478x 104

7.451x

l0a

5.714x ^10-a

5.353 x 10-a

5.047 x 10-a

4.239 x 10-a

3.769 x 10'a

0

1.857 x 10-s

5.681 x l0-5

6.883 x 10-s

9.177 x l0'5

1.453 x 104

1.912

x

l}-a

1.016 x 10-'

9.909 x 10-a

9.822x l0-a

9.516x ^10-a

9.406x l0-a

9.341 x 10-a

9.035 x l0a

8.784x 10-a

0

5.462x 10-6

1.966 x 10-s

2.294x ^10-5

3.059 x 10-s

5.135 x 10-5

7.757 x 10-s

22.5

26.4

31.8

56.5

79.3

u 2.00 x 10-3 M of o-cyclodextrin as host in the aqueous phase.

TABLE 2.

Calculations

of

^K1 using Equation (1)

for the

1:1 a-cyclodextrin-I2 complexu

Time

(h)

_[Iz], M _[H.I2], M _[H], M Kt, M-'

3.8

8.3

1.26 x

l0-'

1.42

x I}'s

2.24

x

l}-s

2.62

x

l}-s

2.57

x

I}-s

3.06 x 10-s

3.00 x 10-s

7.15 x

l0-'

l.l2xl}-a L72xI}a

1.80 x 10-a

1.84 x 10-a

1.93 x 104

1.94 x l0-a

1.93 x 10-'

1.89 x 10-3

1.83 x 10-3

1.82 x 10'3

1.81 x 10-3

1.81 x 10-3

1.81 x 10-3

2.9 x

I0' 4.2x

103

4.2x

103

3.8 x 103

3.9 x 103

3.5 x 103

3.6 x 103

22.5

26.4

31.8

56.5

79.3

Average K1

(3.7!0.4)x10'

u Initial concentrations of iodine in the source hexane phase and a-cyclodextrin in the aqueous phase arc

I.02x

^10-3 and 2.00 x 10-3 M respectively.

TABLE 3.

Calculations

of

K2 using Equation (6)

for

the

1:l

a-cyclodextrin-I3- complexu

0.6 1.86 x 10-'

3.50 x l0's

5.35 x 10-s

6.61 x 10-s

7.81 x 10-s

8.58 x 10-s

7.27

x l}'s

1 .15 x 10-"

230

x

t}-a

3.48 x 10-a

3.88 x 10-a

4.13 x 10-a

4.14

x l}'a

4.24

x

I}-a

1.54 x 10-3

1.42

x

l0-3

1.38 x 10-3

1.36 x 10-3

1.36 x 10-3

1.35x 10-3

4.3 x 103

4.6 x 103

4.3 x 103

3.9 x 103

3.6 x 103

4.3 x 103

1.65x10--F

1.5

2.7

3.8

5.6

9.2

33.9

Average K2 (4.1

+0.3)xl0r

u Initial concentrations of iodine in the source hexane phase and cr,-cyclodextrin and potassium iodide in the aqueous phase are 1.02

x

l0'3 _{, 2.00}

x

10-3 and 1 .04

x

10-2 M

respectively.

TABLE 4. Stability constant Kr of 1:1 complexes formed between iodine and o-, _B-

and y-cyclodextrin in water

Cyclodextrin

Kr, M-' ^(This

work)"

Kr, M-' [refs.3,5]"

cr

^(3.7

t0.4)

^x

103

^{1.5 x}

l0*;2x10";2x10';8.3

x

l0';

9.4 x 103

B Q.4x0.2)x102

1.0x102;1.5x102;1.9x103

y (1.4+9.21*gz 8;24;3.5x102

" at27 ^{oC; o} at 25 ^oC

TABLE

5. Stability constant Kz

of

1:1 complexes formed between triiodide anion and o-, _{F- and} y-cyclodextrin and starch in water

K2, M-' ^(This

work)"

^{K2, M-'} [ref.3]"

cr-

cyclodextrin

^{(4.1 + 0.3) x}

10'

^2.3

x

10";3.3 x 10'

B-

cyclodextrin

^{(7 .2}

r

0.5) x

102

^{1 .9 x 103; 2.2}

x

^103;

5.0 x 103

y-

cyclodextrin

^{(2.2 t0.2) x}

102

^{5.0 x 102}

Starch'

^(7.9

t0.5)

x 102

^ at27 ^{oC; b} at 25 ^oC

'

^{Starch treated as a} chain of B-cyclodextrin ^molecules.

Figure captions

Figure 1. U-tube for transport experiment.

Figure 2. Absorbance at 522 nm of iodine in the hexane source phase at various time intervals at27 ^oC (x-axis for the blank and y-axis for in the presence of hosts); I

Iz]o:

1.05 x l0-3 M and [H]6

:

_{2.00x 10-3 M for o-, B- and} y-cyclodextrin, and I.42 glL fot

starch.

Figure 3. Absorbance at 522 nm of iodine in the hexane source phase at various time intervals at27 ^oC (x-axis for the blank and y-axis for in the presence of hosts);

IIz]o:

1.00

x

l0-3

M, tKIl :

_1.00

x

_10-2

M

_{and [H]s}

:

_1.00

x

_l0-3

M

_{for cr-, B- and T-}

cyclodextrin, and 0.83 g/L for starch.

Hexane (source phase

Magnetic bar

Hexane

(receiving phase) Aqueous phase Stopper

Clamp

Frg t Fott

Absorbance (btank)

5a_I

nc)

€)L

I

+.02

-

s

9.8

II GI

-

L aA

-

4a

,

7-CD

Fi3 2

^PaH

,2

Absorbance (blank)

F,g 3 ,Dctl